Purification of propylene oxide

Abstract: Contaminants are removed from impure propylene oxide by fractionating in a first column to remove overhead essentially all of the pentenes and pentanes and most of the oxygen-containing impurities, extractively distilling the first bottoms fraction in a second extractive column using a C₂ to C₆ alkylene glycol extractive distillation agent to form a second overhead fraction comprising propylene oxide, hexenes, hexanes, water, residual quantities of pentenes and pentanes and oxygen-containing impurities, extractively distilling the second overhead fraction in a third column using a C₆ to C₁₀ alkane hydrocarbon extractive distillation agent to form a third bottoms fraction comprising the extractive distillation agent, propylene oxide, hexenes, hexanes, residual quantities of pentenes and pentanes and water, and extractively distilling said third bottoms fraction in a fourth column using a C₆ to C₁₀ alkane hydrocarbon extractive distillation agent to form a fourth overhead fraction consisting essentially of propylene oxide.

Description: This invention relates to the purification of propylene oxide. More particularly, this invention relates to a plural stage distillation process for removing contaminating quantities of impurities from propylene oxide. Still more particularly, this invention relates to a plural stage distillation process for the purification of impure propylene oxide contaminated with impurities including water, propylene, propane, acetaldehyde, methyl formate, propionaldehyde, hexenes, acetone, hexanes, methanol, t-butyl alcohol, pentanes, pentenes, isopropyl alcohol and t-butyl formate. It is known to use alkane hydrocarbons containing 6 to 18 carbon atoms as extractive distillation agents in the purification of propylene oxide. See, for example, US-A-3338800, US-A-3464897, US-A-3607669 and US-A-3843488. US-A-3464897 shows that an alkane such as octane is effective for the removal of 6 carbon atom alkane impurities such as 2-methyl pentane, 4-methyl pentene-1, 2-methyl pentene-1 and 2-methyl pentene-2. US-A-3843488 shows that alkanes containing from 8 to 20 carbon atoms, and preferably from 8 to 10 carbon atoms, such as n-octane, is effective for removing hydrocarbon impurities containing 5 to 7 carbons from propylene oxide. It is also known to use alkylene glycols containing from 2 to 6 carbon atoms as extractive distillation agents in the purification of propylene oxide. US-A-3578568 discloses the use of ethylene glycol and propylene glycol as extractive agents for the purification of propylene oxide. US-A-5139622 discloses the use of triethylene glycol as an extractive distillation agent and US-A-5160587 discloses the use of dipropylene glycol as an extractive distillation agent. The use of plural stage distillation for the purification of propylene oxide has also been proposed. US-A-3881996 discloses a plural stage process for the purification of propylene oxide including a first fractionation wherein light impurities such as acetaldehyde are removed overhead followed by a second distillation step wherein heavy impurities such as propionaldehyde are removed in order to provide a second distillate fraction which is then extractively distilled in the presence of octane in a third distillation zone to provide pure propylene oxide and to remove alkane hydrocarbon impurities such as C 6 carbon atom impurities from the propylene oxide. Schmidt et al. teach that it is important to use the proper sequence of distillation steps and that, for example, removal of heavy impurities such as propionaldehyde before the removal of light impurities such as acetaldehyde will lead to adverse results. US-A-5133839 discloses a plural stage process for the purification of propylene oxide utilizing a conventional distillation zone which preferably contains two distillation columns, followed by extractive distillation of the parts of purified propylene oxide in two sequential extractive distillation columns using either isoctane or a lower alkylene glycol as the extractive distillation agent and removing lighter impurities in the first of the extractive distillation columns and heavier impurities from the second of the extractive distillation columns. US-A-4971661 discloses a plural stage process for the purification of propylene oxide, but different extractive distillation agents such as water and acetone are used. As illustrated by the prior art just discussed, it was known prior to the present invention to use plural stage distillation for the purification of propylene oxide and to use lower alkylene glycols and higher alkanes as extractive distillation agents. Thus, it was known that the use of the 6 to 10 carbon atom alkanes as extractive distillation agents was effective for removing contaminating quantities of hydrocarbons such as hydrocarbons containing 5 to 7 carbon atoms from propylene oxide. It was also known to use the C 2 -C 6 lower alkylene glycols, which are polar solvents, for the extractive distillation in order to remove oxygenated impurities. Pentanes and pentenes are present in crude propylene oxide as minor impurities. It has been discovered in accordance with the present invention that the removal of pentanes and pentenes presents a difficult problem. The sequence of distillation steps employed in the plural stage purification of propylene oxide is crucial to effective purification. Reversing or otherwise altering the sequence of steps can lead to adverse results. Pentanes and pentenes may be removed like other hydrocarbon impurities by using octane in an extractive distillation. The octane lowers the relative volatility of the hydrocarbons relative to propylene oxide. However, the degree of relative volatility change is proportional to the amount of octane solvent present. Using typical solvent to feed ratios as disclosed in the prior art, it has been discovered that pentanes and pentenes are not easily removed and tend to remain in the PO product. Therefore, this invention discloses that a more economical and efficient means of removing pentanes and pentenes is by a conventional distillation upstream of the octane extractive distillation. The C5 stripper should not be located downstream of the octane extractive distillation since the final PO product would then be a bottoms product off the stripper. It is advantageous to take high purity products off the top of columns rather than off the bottom. Thus, when a partially purified propylene oxide containing contaminating quantities of 5 to 7 carbon atom hydrocarbons such as pentanes, pentenes, hexanes, hexenes, heptanes and heptenes is subjected to extractive distillation using an alkane hydrocarbon extractive distillation agent such as octane, a significant portion of the pentanes and pentenes will remain with the overhead purified propylene oxide rather than being removed therefrom for withdrawal from the bottom of the tower together with the extractive distillation agent and the other impurities. As a consequence, a subsequent distillation step would normally be required for the removal of the pentanes and pentenes. It has been discovered in accordance with the present invention, however, that the use of a different sequence of distillation steps from that disclosed in the prior art will obviate this problem so that there is no need for an additional distillation tower downstream of the alkane hydrocarbon extractive distillation. In accordance with the present invention, contaminating quantities of hexenes, hexanes, pentenes, pentanes, water and oxygen-containing impurities are removed from impure propylene oxide by fractionating the impure propylene oxide in a first distillation column to provide a first overhead fraction comprising essentially all of the pentenes and pentanes and most of the oxygen-containing and hydrocarbon impurities boiling above propylene oxide, including isobutylene, and a partially purified first propylene oxide bottoms fraction comprising propylene oxide, hexenes, hexanes, and only residual quantities of pentenes and pentanes, extractively distilling the first propylene oxide bottoms fraction in a second extractive distillation column in the presence of a C 2 to C 6 alkylene glycol extractive distillation agent to form a second overhead fraction comprising propylene oxide, hexenes, hexanes, water, methanol, residual quantities of pentenes and pentanes and oxygen-containing impurities boiling above propylene oxide, and a second bottoms fraction comprising the extractive distillation agent, water, methanol and oxygen-containing impurities boiling below propylene oxide, extractively distilling the second overhead fraction in a third extractive distillation column in the presence of a C 6 to C 10 , preferably C 7 to C 10 alkane hydrocarbon extractive distillation agent to form a third overhead fraction comprising the extractive distillation agent and propylene oxide enriched with impurities boiling above propylene oxide and water and a third bottoms fraction comprising the extractive distillation agent, propylene oxide, hexenes, hexanes, residual quantities of pentenes and pentanes, and extractively distilling said third bottoms fraction in a fourth extractive distillation column in the presence of a C 6 to C 10 , preferably C 7 to C 10 , alkane hydrocarbon extractive distillation agent to form a fourth overhead fraction consisting essentially of propylene oxide and a fourth bottoms fraction comprising the extractive distillation agent, hexenes, hexanes residual quantities of pentenes and pentanes. It has also been discovered in accordance with the present invention that any minor quantity of isobutylene that is present in the impure propylene oxide is substantially completely removed together with the pentenes and pentanes in the first distillation column. It has been further discovered that this facilitates water removal in the second extractive distillation column. It is believed that the distillation conditions that are normally used in the second distillation column tends to promote minor thermal decomposition of the tertiary butyl alcohol to isobutylene and water. Therefore, it is important to minimize the isobutylene content of the feed to the second distillation column in order to minimize the water content of the overhead fraction taken therefrom. In accordance with a preferred embodiment of the present invention, impure propylene oxide contaminated with hexenes, hexanes, pentenes, pentanes, isobutylene, water and oxygen-containing impurities is purified by a plural stage distillation process which includes the steps of: (a) charging the impure propylene oxide to a first distillation column under distillation conditions selected to provide a first overhead distillation fraction comprising essentially all of the pentenes, pentanes and isobutylene and most of the oxygen-containing and hydrocarbon impurities boiling above propylene oxide and a partially purified first propylene oxide bottoms fraction comprising propylene oxide, hexenes, hexanes, and only residual quantities of pentenes and pentanes, (b) charging the first propylene oxide bottoms fraction to a second extractive distillation column separately charging a C 2 to C 6 alkylene glycol extractive distillation agent at a point above the point of introduction of the first propylene oxide bottoms fraction (an ascending charge point), and separating said first propylene oxide bottoms fraction therein into a second overhead fraction comprising propylene oxide, hexenes, hexanes, residual quantities of pentenes, pentanes, isobutylene and water, methanol and oxygen-containing impurities boiling above propylene oxide, and a second bottoms fraction comprising the extractive distillation agent, water, methanol and oxygen-containing impurities boiling below propylene oxide, (c) separately charging the second overhead fraction and a C 6 to C 10 , preferably C 7 to C 10 alkane hydrocarbon extractive distillation agent to a third extractive distillation column at ascending charge points and separating said second overhead fraction therein into a third overhead fraction comprising oxygen-containing impurities boiling above propylene oxide and a third bottoms fraction comprising the extractive distillation agent, propylene oxide, hexenes, hexanes residual quantities of pentenes and pentanes, and (d) separately charging the third bottoms fraction and a C 6 to C 10 , preferably C 7 to C 10 alkane hydrocarbon extractive distillation agent to a fourth extractive distillation column at ascending charge points and separating said third bottoms fraction therein into a fourth overhead fraction consisting essentially of propylene oxide and a fourth bottoms fraction comprising the extractive distillation agent, hexenes, hexanes, residual quantities of pentenes and pentanes. In accordance with the preferred embodiment of the present invention, impure propylene oxide contaminated with isobutylene, hexenes, hexanes, pentenes, pentanes, water and oxygen- and hydrogen-containing impurities including propylene, propane, acetaldehyde, methyl formate, propionaldehyde, acetone, methanol, tertiary butyl alcohol, isopropyl alcohol, and tertiary butyl formate is purified by a plural stage distillation process which includes the steps of: (a) fractionating said impure propylene oxide in a first distillation column under distillation conditions selected to provide a first overhead distillation fraction comprising essentially all of the isobutylene, pentenes and pentanes and most of the oxygen-containing and hydrocarbon impurities boiling above propylene oxide including the propylene, propane, acetaldehyde, and a partially purified first propylene oxide bottoms fraction comprising propylene oxide, hexenes, hexanes, and only residual quantities of pentenes and pentanes, (b) separately charging said first propylene oxide bottoms fraction and a C 2 to C 6 alkylene glycol extractive distillation agent to a second extractive distillation column at ascending charge points and separating said first propylene oxide bottoms fraction therein into a second overhead fraction comprising propylene oxide, hexenes, hexanes, residual quantities of pentenes and pentanes, acetaldehyde, methyl formate, methanol and a minor amount of isobutylene and water formed in the column, and a second bottoms fraction comprising the extractive distillation agent, water, methanol and oxygen-containing impurities boiling below propylene oxide, (c) separately charging said second overhead fraction and a C 6 to C 10 alkane hydrocarbon extractive distillation agent to a third extractive distillation column at ascending charge points and separating said second overhead fraction therein into a third overhead fraction comprising oxygen-containing impurities including acetaldehyde, methyl formate, methanol and water and a third bottoms fraction comprising the extractive distillation agent, propylene oxide, hexenes, hexanes, residual quantities of pentenes and pentanes; and (d) separately charging said third bottoms fraction and a C 6 to C 10 , preferably a C 7 to C 10 alkane hydrocarbon extractive distillation agent to a fourth extractive distillation column at ascending charge points and separating said third bottoms fraction therein into a fourth overhead fraction consisting essentially of propylene oxide and a fourth bottoms fraction comprising the extractive distillation agent, hexenes, hexanes, residual quantities of pentenes and pentanes. The invention will be further described with reference to the accompanying drawing, which is a schematic flow sheet with conventional parts omitted showing the general recovery sequence that is used in accordance with the present invention to purify propylene oxide. In the drawing, conventional parts such as valves, pumps, temperature sensors, pressure sensors, heaters, coolers, flow control regulation apparatus, reboilers and reflux condensers have been omitted. The impure propylene oxide to be purified in accordance with the present invention is typically propylene oxide prepared by the reaction of tertiary butyl hydroperoxide with propylene in the presence of a molybdenum catalyst to provide a reaction mixture comprising unreacted propylene, unreacted tertiary butyl hydroperoxide, tertiary butyl alcohol, propylene oxide, and impurities. This reaction product is separated in a distillation zone (not shown) into a plurality of fractions including a propylene recycle fraction, an impure propylene oxide fraction, a tertiary butyl alcohol fraction and a residue fraction. The impure propylene oxide obtained in this fashion is suitably used as a feedstock for the present invention and will normally be contaminated with impurities including water, isobutylene, propylene, propane, acetaldehyde, methyl formate, propionaldehyde, hexenes, hexanes, acetone, methanol, tertiary butyl alcohol, pentanes, pentenes, isopropyl alcohol and tertiary butyl formate. The impure propylene oxide will normally contain from 97 to 99.5 wt.% of propylene oxide, the balance being impurities such as those enumerated above. It will be understood that some of the enumerated impurities will not always be present in every batch of impure propylene oxide and that other impurities not listed may be present in minor quantities. The impurities, broadly speaking, comprise water, hydrocarbons such as propylene, propane, isobutylene, hexenes, hexanes, pentane, isopentane and pentenes, and oxygenated impurities including aldehydes, alcohols, esters, etc. The hydrocarbon impurities will normally contain from 2 to 6 carbon atoms and the oxygenated impurities will normally contain from 1 to 6 carbon atoms also. In accordance with the present invention, the impure propylene oxide is charged to a first distillation column 20 which may suitably contain from about 20 to about 40 theoretical trays, the feedstock being suitably charged to 8 to 16 theoretical trays from the bottom of the tower. Distillation conditions are adjusted in the distillation column 20 to provide a first lighter distillation fraction discharged by way of a line 22 and a first heavier distillation fraction discharged by a line 24. Distillation conditions are suitably adjusted in the distillation column 20 so that substantially all of the propylene oxide will be present in the first heavier distillation fraction 24. Typically, the lighter distillation fraction 22 will comprise impurities including propylene, propane, isobutylene, acetaldehyde and, significantly, substantially all of the pentanes and pentenes charged to the distillation column 20. The first heavier distillation fraction will typically contain substantially all of the propylene oxide, hexenes, hexanes and only residual quantities of pentanes and pentenes. The first bottoms fraction is charged to a second extractive distillation column 10 by way of a charge line 24. The fractional distillation column 10 may suitably comprise from 40 to 80 theoretical trays and the charge line 24 for the impure propylene oxide will normally be at least 10 to 30 trays from the bottom of the distillation tower. An alkylene glycol extractive distillation agent is also charged to the second extractive distillation column 10 by a line 14. The alkylene glycol extractive distillation agent may suitably be an alkylene glycol containing from 2 to 6 carbon atoms such as ethylene glycol, propylene glycol, 1,3-propane diol, 1,4-butane diol, 2-methyl-1,3-propane diol, diethylene glycol, triethylene glycol and dipropylene glycol. Preferred extractive distillation agents include triethylene glycol and dipropylene glycol. The alkylene glycol extractive agent 14 will suitably be charged to the extractive distillation column 10 in the ratio of 3 to 7 parts of impure propylene oxide per part of alkylene glycol extractive distillation agent. Extractive distillation conditions are adjusted in the extractive distillation tower 10 so as to provide for the recovery overhead or as a distillate fraction of substantially all of the propylene oxide charged to the extractive distillation column 10. For example, the impure propylene oxide, which will suitably comprise 98.5 wt.% propylene oxide, the balance being impurities as mentioned above, and may be charged at a temperature of 39°C-71°C (100°F-160°F). The temperature at the bottom of the extractive distillation column 10 may suitably be 149°C-193°C (300-380°F) and the pressure may be about 138-345 kPa (20-50 psia). The temperature at the top of the extractive distillation column 10 may, for example, be 32°-54°C (90-130°F) and the pressure may suitably be 69-27 kPa (10-30 psia). A second lighter distillation fraction comprising impure propylene oxide is removed from the column 10 by way of a line 16 and may suitably comprise more than 99 wt.% propylene oxide, the balance being impurities including acetaldehyde, methyl formate, hexenes, hexanes, methanol, water, and residual pentanes, isopentanes and pentenes. A second bottom distillation fraction 18 discharged adjacent the bottom of the extractive distillation column 10 will comprise impurities including water, propionaldehyde, acetone, methanol, tertiary butyl alcohol, isopropyl alcohol, tertiary butyl formate and the alkylene glycol extractive distillation agent. The second overhead fraction 16 is charged to a third distillation column 30 together with a C 6 to C 10 alkane hydrocarbon extractive distillation agent which is charged by a line 32. The alkane extractive distillation agent may suitably be charged to the third distillation column 30 in the ratio of 5 to 7 parts of extractive distillation agent per part of the second overhead fraction 16. Extractive distillation conditions are adjusted in the distillation column 30 to provide for a third heavier distillation bottoms fraction 33 containing substantially all of the propylene oxide, hexenes, hexanes, and residual pentenes and pentanes and a lighter third overhead distillation fraction 34 will typically be comprised of the C 7 to C 10 hydrocarbon extractive distillation agent, and propylene oxide enriched with impurities such as methyl formate, acetaldehyde, water and methanol. Residual quantities of hexane may also be present. The extractive distillation agent charged to the distillation column 30 is suitably an 8 carbon atom alkane such as normal octane or isoctane. A minor amount of alkanes such as nonane (usually less than 1 wt.%) may also be present in the extractive distillation agent charged to the extractive distillation tower 30 by the line 32. The third feedstock 16 charged to the third distillation column 30 may be suitably charged at a temperature of about 54°-82°C (130-180°F). The temperature at the bottom of the extractive distillation column 30 may suitably be about 54°-104°C (180-220°F) and the pressure may suitably be 207-344 kPa (30-50 psia). The temperature adjacent the top of the extractive distillation tower 30 may suitably be 54°-71°C (130-160°F) and the pressure may suitably be 138-276 kPa (20-40 psia). The third heavier distillation bottoms fraction 33 is charged to a fourth extractive distillation tower 40 together with an extraction distillation agent such as octane which is charged by way of a line 42. The extractive distillation agent 42 may be charged to the extractive distillation column 40 in the ratio of 0.4 to 0.8 parts of extractive distillation agent per part of fourth feedstock 33. Distillation conditions are adjusted in the distillation column 40 to provide lighter fourth overhead distillation fraction 44 consisting essentially of propylene oxide and a heavier fourth bottoms distillation fraction 46 comprising the hexenes, hexanes, residual pentenes, residual pentanes and the extractive distillation agent. Suitably, the temperature at the top of the distillation column 40 may be 38°-54°C (100-130°F) and the pressure may be 69-207 kPa (10-30 psia). The temperature adjacent the bottom of the distillation column 40 may suitably be 121°-143°C (250-290°F) and the pressure may suitably be 173-310 kPa (25-45 psia). The second heavier distillation bottoms fraction 18 from the second distillation column 10 may be charged to a fifth distillation column 50 containing from 20 to 60 theoretical trays wherein the fifth feedstock may be separated under distillation conditions adjusted to provide for the recovery of substantially all of the alkylene glycol extractive distillation agent as a heavier distillation fraction discharged from the distillation column by a line 14 and a lighter distillation fraction 54 discharged from the distillation column 50 and suitably containing substantially all of the water, acetone, tertiary butyl alcohol, methanol and isopropyl alcohol contained in the fifth feedstock 18. The pressure at the top of the fifth distillation column 50 may suitably be 0.7-7 kPa (0.1-1.0 psia) and the temperature may suitably be 32°-54°C (90-130°F). The pressure at the bottom of the distillation column 50 may suitably be 0.7-7.5 kPa (0.1-1.1 psia) and the temperature may suitably be 149°-182°C (300-360°F). The third lighter distillation fraction 34 and the fourth heavier distillation fraction 46 may suitably be charged by way of a line 48 to a sixth distillation column 60 as a sixth feedstock which may comprise, for example, from 30 to 50 theoretical trays and wherein the sixth feedstock is introduced at least about 20 theoretical trays from the bottom of the column. Distillation conditions are adjusted within the distillation column 60 to provide for the recovery of a sixth lighter distillation fraction 62 comprising acetaldehyde, methyl formate, propylene oxide, hexenes, methanol and a minor amount of octane and a sixth heavier distillation fraction 64 comprising substantially all of the extractive distillation agent charged to the distillation column 60 by the line 48. For example, the distillation conditions established in the sixth distillation column 60 may include a pressure at the top of the column of 103-206 kPa (15-30 psia) and a temperature of 38°-60°C (100-140°F) and a pressure at the bottom of the column of 138-276 kPa (20-40 psia) and a temperature of 93°-149°C (200-300°F). EXAMPLES The invention will be further illustrated by the following specific example which is given by way of illustration and not as a limitation on the scope of this invention. Where parts are mentioned, they are parts by weight. EXAMPLE 1 About 1000 parts of an impure propylene oxide feedstock 12 containing about 98.5 wt.% of propylene oxide and contaminated with impurities including water, propylene, propane, acetaldehyde, methyl formate, propionaldehyde, hexenes, acetone, hexanes, methanol, t-butyl alcohol, n-pentane, isopentane (i.e., 2-methyl butane), pentene, isopropyl alcohol and tertiary butyl formate is charged to a first distillation column 20 where it is separated into about 999 parts of a first heavier distillation bottoms fraction 24 and about 1 parts of a first lighter distillation fraction 22 containing isobutylene, propylene, propane, acetaldehyde, butanes, butenes, pentenes and pentanes. The first bottoms fraction 24 is charged to a second extractive distillation column 10 by a line 24 and is separated under the extractive distillation conditions in the presence of a triethylene glycol extractive distillation agent into a second lighter overhead distillation fraction 16 (about 986 parts) containing more than about 99 wt.% of propylene oxide and less than about 1 wt.% of contaminants including propylene, propane, acetaldehyde, methyl formate, hexenes, hexanes, pentenes, pentanes, water and methanol and a second heavier distillation bottoms fraction containing water, propionaldehyde, acetone, tertiary butyl alcohol, tertiary butyl formate, isopropyl alcohol, methanol and the triethylene glycol extractive distillation agent. The second heavier distillation fraction 18 is separated in fifth distillation column 50 into a heavier fraction 14 comprising the triethylene glycol extractive distillation agent which is recycled to the extractive distillation column 10 and a lighter overhead fraction 54 comprising water, acetone, tertiary butyl alcohol, methanol and isopropyl alcohol. The second lighter overhead fraction 16 discharged from the distillation column 10 will comprise substantially all of the propylene oxide, hexenes, hexanes, and only residual quantities of pentanes and pentenes. This fraction is charged by way of the line 16 to a third distillation column 30 together with about 5620 parts of octane charged to the distillation column 30 by a line 32. The distillation column 30 is operated in the manner described above to provide a third lighter distillation overhead fraction 34 (about 13 parts) and a third heavier bottoms distillation fraction 32 (about 6593 parts). The fraction 34 will comprise acetaldehyde, methyl formate, methanol, water, propylene oxide and octane. The third heavier bottoms distillation fraction 33 will comprise propylene oxide, hexanes and substantially all of the octane charged to the third distillation column 30. The third heavier bottoms distillation fraction 33 is charged to the fourth extractive distillation column 40 together with about 3690 parts of octane where it is separated in the manner described above into about 975 parts of an overhead distillation fraction consisting essentially of propylene oxide and containing very low quantities of the impurities present in the feedstock charged by the line 12 to the first distillation column 20. A heavier distillation fraction 46 discharged from the column 40 will comprise about 9309 parts of octane, hexenes, hexanes and residual quantities of propylene oxide. EXAMPLE 2 A plural stage distillation purification of contaminated propylene oxide was conducted over a one week period. The water content in the second overhead distillation column was in the range of about 40 to 100 parts per million. The water content of the fourth overhead distillation fraction was in the range of about 10 to 30 ppm. In a comparative study, the distillation sequence was altered so that the contaminated propylene oxide was first extractively distilled in the column 10 and then distilled in the column 20. The test was conducted over a one week period under substantially identical distillation conditions. The water content in the second overhead distillation column was in the range of about 100 to 200 parts per million. The water content of the fourth overhead distillation fraction was in the range of about 15 to 100 ppm.

Claims: A process for separating contaminating quantities of hexenes, hexanes, pentenes, pentanes, butenes, water and oxygen-containing impurities from impure propylene oxide which comprises: (a) fractionating said impure propylene oxide in a first distillation column under distillation conditions selected to provide a first overhead distillation fraction comprising essentially all of the pentenes, pentanes and butenes and most of the oxygen-containing and hydrocarbon impurities boiling above propylene oxide and a partially purified first propylene oxide bottoms fraction comprising propylene oxide, hexenes, hexanes, and residual quantities of pentenes and pentanes, (b) extractively distilling said first propylene oxide bottoms fraction in a second extractive distillation column in the presence of a C 2 to C 6 alkylene glycol extractive distillation agent to form a second overhead fraction comprising propylene oxide, hexenes, hexanes, water, residual quantities of pentenes and pentanes and oxygen-containing impurities boiling above propylene oxide, and a second bottoms fraction comprising the extractive distillation agent, water, methanol and oxygen-containing impurities boiling below propylene oxide, (c) extractively distilling said second overhead fraction in a third extractive distillation column in the presence of a C 6 to C 10 alkane hydrocarbon extractive distillation agent to form a third overhead fraction comprising the extractive distillation agent and propylene oxide enriched with the oxygen-containing impurities boiling above propylene oxide and a third bottoms fraction comprising the extractive distillation agent, propylene oxide, hexenes, hexanes, residual quantities of pentenes and pentanes; and (d) extractively distilling said third bottoms fraction in a fourth extractive distillation column in the presence of a C 6 to C 10 alkane hydrocarbon extractive distillation agent to form a fourth overhead fraction consisting essentially of propylene oxide and a fourth bottoms fraction comprising the extractive distillation agent, hexenes, and hexanes. A process as claimed in Claim 1, wherein isobutylene is also present as a contaminant, essentially all of the isobutylene being distilled to the first overhead distillation fraction, with only residual quantities in the bottoms fraction. A process as claimed in Claim 1 or Claim 2, for the purification of impure propylene oxide contaminated with isobutylene, hexenes, hexanes, pentenes, pentanes, water and oxygen- and hydrogen-containing impurities including propylene, propane, acetaldehyde, methyl formate, propionaldehyde, acetone, methanol, tertiary butyl alcohol, isopropyl alcohol, and tertiary butyl formate which comprises the steps of: (a) fractionating said impure propylene oxide in a first distillation column under distillation conditions selected to provide a first overhead distillation fraction comprising essentially all of the isobutylene, pentenes and pentanes and most of the oxygen- and hydrogen-containing impurities boiling above propylene oxide including the propylene, propane, acetaldehyde, and substantially all of the pentenes and pentanes and a partially purified first propylene oxide bottoms fraction comprising propylene oxide, hexenes, hexanes, and only residual quantities of pentenes and pentanes and isobutylene, (b) separately charging said first propylene oxide bottoms fraction and a C 2 to C 6 alkylene glycol extractive distillation agent to a second extractive distillation column at ascending charge points and separating said first propylene oxide bottoms fraction therein into a second overhead fraction comprising propylene oxide, hexenes, hexanes, residual quantities of pentenes and pentanes, acetaldehyde, methyl formate, methanol and isobutylene and water, and a second bottoms fraction comprising the extractive distillation agent, water, methanol and other oxygen-containing impurities boiling below propylene oxide, (c) separately charging said second overhead fraction and a C 6 to C 10 alkane hydrocarbon extractive distillation agent to a third extractive distillation column at ascending charge points and separating said second overhead fraction therein into a third overhead fraction comprising the extractive distillation agent and propylene oxide enriched in oxygen-containing impurities including acetaldehyde, methyl formate, methanol and water and a third bottoms fraction comprising the extractive distillation agent, propylene oxide, hexenes, hexanes, residual quantities of pentenes and pentanes and water; and (d) separately charging said third bottoms fraction and a C 6 to C 10 alkane hydrocarbon extractive distillation agent to a fourth extractive distillation column at ascending charge points and separating said third bottoms fraction therein into a fourth overhead fraction consisting essentially of propylene oxide and a fourth bottoms fraction comprising the extractive distillation agent, hexenes, hexanes, residual quantities of pentenes and pentanes and water. A method as claimed in anyone of Claims 1 to 3, wherein the first distillation column contains 20 to 40 theoretical trays, and wherein the impure propylene oxide is introduced at least 8 trays from the bottom of the column; and/or wherein the second distillation column contains 40 to 80 theoretical trays, wherein the first propylene oxide bottoms fraction is introduced at least 10 trays from the bottom of the column, wherein the C 2 to C 6 alkylene glycol extractive distillation agent is introduced at least 30 theoretical trays above the tray at which the first propylene oxide bottoms fraction is introduced, and wherein the weight ratio of first feedstock to the extractive distillation agent is 6:1 to 3:1; and/or wherein the third distillation column contains 50 to 80 theoretical trays, wherein the second overhead fraction is introduced at least 30 trays from the bottom of the column, wherein the C 6 to C 10 alkane hydrocarbon distillation agent is introduced at least 10 theoretical trays above the tray at which the second overhead fraction is introduced, and wherein the weight ratio of second overhead fraction to the extractive distillation agent is 0.1 to 0.3; and/or wherein the fourth distillation column contains 40 to 80 theoretical trays, wherein the third bottoms fraction is introduced at least 3 trays from the bottom of the column, wherein the C 6 to C 10 alkane hydrocarbon extractive distillation agent is introduced at least about 15 theoretical trays above the tray at which the third bottoms fraction is introduced, and wherein the weight ratio of third bottoms fraction to the extractive distillation agent is 1.3 to 2.2. A method as claimed in claim 3 wherein the second heavier distillation fraction is charged to a fifth distillation column and fractionated therein under distillation conditions selected to provide a fifth lighter distillation fraction containing substantially all of the water, acetone, tertiary butyl alcohol, methanol and isopropyl alcohol charged to the fifth distillation column and a fifth heavier distillation fraction containing substantially all of the alkylene glycol extractive distillation agent and its reaction products charged to the fifth distillation column, and wherein said fifth heavier distillation fraction is recycled to said second distillation column as said alkylene glycol extractive distillation agent and its reaction products. A method as claimed in claim 3 wherein a sixth feedstock comprising said third lighter distillation fraction and said fourth heavier distillation fraction is charged to a sixth distillation column, and wherein said sixth feedstock is fractionated in said sixth distillation column under distillation conditions selected to provide a sixth lighter distillation fraction containing substantially all of the acetaldehyde, methyl formate, propylene oxide, hexenes, hexanes, methanol and a minor portion of the C 6 to C 10 alkane hydrocarbon extractive distillation agent charged to the sixth distillation column and a sixth heavier distillation fraction containing substantially all of the C 6 to C 10 alkane hydrocarbon extractive distillation agent charged to the third and fourth distillation columns, and wherein said sixth heavier distillation fraction is recycled to said third and fourth distillation columns as said third and fourth extractive distillation agents.

Inventors: CULBRETH III WILLIAM KEMP, POTTRATZ DAVID GEORGE, SHEU YA-HWA EDWARD, CULBRETH III, WILLIAM KEMP, POTTRATZ, DAVID GEORGE, SHEU, YA-HWA EDWARD
Applicants: TEXACO CHEM INC, TEXACO CHEMICAL INC.
Application year: 1994
Main classification: C07D301
IPC codes: B01D3, C07D301, C07D303